The present invention relates to a storage system, and more particularly to data replication among a plurality of storage systems. A technology relating to data replication among storage systems becomes important for providing uninterrupted service to customers even when failure occurs in a first storage system. The following patent specifications have disclosed a technology for replicating the information stored in a first storage system to a second and third storage system.
U.S. Pat. No. 5,170,480 discloses a technology by which a first computer connected to a first storage system transfers data stored in a first storage system to a second computer via a communication link between a first computer and the second computer, and the second computer transfers the data to a second storage system connected to the second computer.
U.S. Pat. No. 6,209,002 discloses a technology by which a first storage system transfers data stored in a first storage system to a second storage system and the second storage system transfers the data to a third storage system. A computer is connected to the first storage system by a communication link, the first storage system is connected to the second storage system by a communication link, and the second storage system is connected to the third storage system by a communication link. The first storage system holds a first logical volume which is the replication object. The second storage system holds a second logical volume which is a duplicate of the first logical volume and a third logical volume which is a duplicate of the second logical volume. The third storage system holds a fourth logical volume which is a duplicate of the third logical volume. As described in the patent specification, the second storage system executes the process for data replication exclusively from the second logical volume to the third logical volume and the process for data replication from the third logical volume to the fourth logical volume.
With the technology disclosed in U.S. Pat. No. 5,170,480, the first and second computers are used all the time. The first computer conducts the usual operations and the load of the data replication process on the first computer cannot be ignored. Another problem is extending the time for data reference and data update necessary for normal operation. Because the data for replication use a communication link between the first computer and the first storage system, this data transfer collides with the data transfer necessary for normal operation.
With the technology disclosed in U.S. Pat. No. 6,209,002, a storage capacity twice as large as the quantity of the replicated data is required for the second and third storage systems. Furthermore, because a large quantity of the data is the object of replication, a long time is spent on the data replication process and the data of the third storage system becomes obsolete. As a result, a problem arises when the operation is restarted using the data of the third storage system: a long time is required to update the data of the third storage system and the time to restart the operation is extended. Furthermore, as described in this specification, after the first storage system has performed process of data updating in the first storage system and process of data advancement between the first storage system and the second storage system, the first storage system sends a data update completion report to the host computer. Therefore, a long time is spent on updating data from the computer, and the time spent on updating data increases with the distance between the first storage system and the second storage system. As a result, another problem associated with the technology disclosed in the '002 patent is that the distance between the storage systems cannot be increased significantly.